Minor bugfix in sample code multipart.asciidoc

[ahron1]

For reading a multipart message where the part is a file :-

this line below leads to a badmap error after the last chunk has been read. So the functionality isn't affected but it just throws an error at the very end. Req = case cow_multipart:form_data(Headers) of

Removing the pattern matching and replacing it with just case cow_multipart:form_data(Headers) of

resolves the issue. Doing this means the recursive call multipart(Req) should be replaced with multipart(Req1).